2004 Chevy Classic MISFIRE

I have a 2004 Chevy Classic with about 235,000 miles on it. Up until this point, it’s been a reliable work car. I got into the car this morning and upon starting it and idling I had a VERY noticeable misfire. The check engine light was on. As soon as I backed out of the driveway and started driving the CEL started flashing confirming my suspicions. The car shakes and the exhaust makes a constant rhythmic putting noise and it gets worse accelerating, especially from a dead stop.

I am far from a car expert and my knowledge on car repair is minimal. I also own pretty much no automotive repair tools except for a multimeter and fuel pressure tester I got hold of today. I took the car to autozone to borrow a scan tool to get freeze frame data since I don’t have my own.

P0172 System Too Rich (Bank 1)
P0302 Cylinder 2 Misfire Detected
P0442 Evaporative Emission System Leak Detected (small leak)
P0302 Cylinder 2 Misfire Detected

Fuel Pressure Readings
KOEO Prime: 54 PSI
15 minutes later 51 PSI (I ruled out an injector leak based on this finding, is this reasonable?)
Turning on the car it idles at 60 PSI
When revving the engine up it stayed at 60 psi, the misfire was still present.
I initially thought these readings were high based on what others say is typical for other vehicles but I know every engine is different and I don’t have the specs for this one so I don’t know either way.

I removed the vacuum line from what I assumed was the regulator on the end of the fuel rail and noticed there was no vacuum (used finger, not gauge). I used one of those hand pump vac gauges to manually pull a vacuum on the regulator and it did cause PSI to drop, but had no effect on the misfire.

I have no idea where to go and need direction, could you guys please help me?

  • One of the biggest signs your cylinder is misfiring is the loss of power along with odd vibrations. Since the cylinder powers the engine, you will notice your fuel economy start to suffer since the remaining working cylinders have to compensate for the loss of power. In addition, if your vehicle shakes while idling this is another sign of a misfire. Combine these, and they are sure signs your cylinder is misfiring and needs to be looked at by a mechanic as soon as possible.
  • Another reason a cylinder misfires is because of a loss of spark. This can be something that stops coil voltage from hopping the gap at the end of the spark plug. Damaged, worn, or bad spark plugs or a weak ignition coil can cause a loss of spark, and therefore, a misfiring cylinder.
  • If there is not enough gasoline in the air/fuel mixture, this can cause a misfire as well. If the fuel injector is clogged, dirty, or has an air leak, the low pressure will affect all of the cylinders, rather than just one cylinder.
  • Sometimes cylinders have intermittent misfires, which means the cylinder does not misfire all the time. The misfire may happen when it is cold out or while the vehicle is carrying a heavy load. Other times it may seem like the cylinder misfires randomly, with no pattern. These are difficult issues to diagnose, so the vehicle needs to be looked at by a professional mechanic.

Driving with a misfiring cylinder is potentially dangerous. If you lose power while you are driving, or a second or third cylinder goes out, this can cause you to get into a car accident, potentially injuring you and others around you. If you suspect a misfiring cylinder, make an appointment with a mobile technician as soon as possible to have your vehicle inspected.

With 235K on the injectors… they may not be leaking (your test proved that, Thank You) but cylinder #2s injector may not be operating properly. It may be hanging open and causing the misfire. OR the #2 spark plug or coil way also not be firing causing raw fuel to be dumped into the exhaust.

I have the tools to run a compression test and I’d do that first thing here but that may be a bit much to ask for you. Here are some quick and cheap things you can check first. Whenever you get to the point of…Whoa, no, not doing that…take it to a good independent repair shop. Avoid chains.

I’d suggest removing the #2 spark plug and looking at its condition. Google “sparkplug condition chart” for a picture. That will hint at the condition of the cylinder itself. Replace just that plug, for now. See if the P0302 and the P0172 go away. If it goes away for a while and then comes back in the same cylinder, the rings or valve seal may be oil fouling that plug telling you the engine is wearing out.

If the problem stays, try swapping the coil from another cylinder. Does the misfire move? Or stay? If it moves, the coil is bad, replace it. Consider replacing all the plugs and spark plug wires (if the engine has any)

If it is still cylinder #2 misfiring, it isn’t the sparkplug, it isn’t the coil or wire soooo, maybe the injector is hanging open. Buy a set of O-rings for the injectors. Remove the injectors and fuel rail as an assembly. swap injector #2 with its neighbor. Install new O-rings on each injector. They are so old, the O-rings WILL leak if you don’t replace them all at this point. Re-install the rail. Does the misfire move to where #2s injector now sits? Replace the injector… or ALL the injectors… rebuilt ones would be OK.

The P0442 is a separate problem, probably a bad gas cap. Don’t worry about that for now.


Thank you kindly for the suggestions. I’ll assess in the order you mentioned, plug/coil/injector. Just a couple concerns though.

I pulled this image off google. Am I correct in the numbering of the cylinders here?

Also, can I swap coils from cylinder to cylinder as you mentioned with a coil assembly like this? It looks like it’d be one unit. Or is there a different way to test it.

The cylinder numbering is correct. You cannot swap coils on on this type of ignition cassette so skip that step or buy a complete unit. Avoid cheap replacements. They are cheap for a reason.

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There was a bracket that holds the throttle cable and cruise control throttle cable in the way of the coil so I had to remove that to finally get the coil out. Upon removing it I noticed there was oil sitting in that recessed area under the coil pack around the spark plug holes. Valve cover gasket/spark plug tube seals?

I removed the spark plug from cylinder 2 and the first thing I noticed was it was dripping wet. I don’t know if I’d say it was straight gasoline but it could have been, then again it could have been the WD40 I sprayed 20 minutes prior. Smelled like the latter more.

Here are a couple pictures of the plug I pulled from the cylinder.

I noticed the large gap there so I figured maybe it would at least make a little difference putting a brand new plug in. I got everything back together started the car and it seemed to make no difference at all. The CEL went away but that’s probably just because I unplugged the battery.
Before I go swapping the entire coil though, I’d like to verify I’m even getting any sort of spark at the missing cylinder and now I’m regretting not doing it while I had it apart. But now that I know how to get it back apart it probably won’t be a big deal. How can I verify I have spark at the suspect cylinder?

The electrodes on that plug are worn out. Replace all four plugs after checking the gap in the new ones. See how the car runs before you try something else.

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Yes. You need to replace those. It’s not that hard.

Are those the original spark plugs? That plug looks like it has 235,000 miles on it. The spark plugs should be replaced every 100,000 miles.

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Yes, you need to replace those. It’s not very hard.

Agree with the others, drive a bit and see if the code comes back. If it doesn’t the coil is ok. That is how you check spark.

Seems like the plugs have never been changed. The other 3 need changing after you confirm the CEL is gone. Buy a valve cover gasket seal kit and do that when the new plugs go in.

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Just drove around the block. Few seconds after leaving the driveway the check engine light came back on. I tried out some moderate acceleration and it started blinking again.

Pull the code and post it, please.

P0302 cylinder 2 misfire detected
Got that code twice

Will be trying this along with cleaning the oil off the throttle body as well if I can get this running properly again.

Replace all 4 plugs, do the valve cover gasket and consider a new coil pack.


I took the coil out, laid it on it’s side to where there was a small gap between the outputs and grounded valve cover. After cranking the engine all had spark except #2 which was completely dead. Will definitely be getting a new coil and plugs. Thanks guys! Hope this solves the rich condition as well, if not I’ll pickup here.